[Intel-gfx] [PATCH] drm/i915: Trim partial view sg lists

[Chris Wilson]

Quoting Tvrtko Ursulin (2018-09-26 09:03:53)
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> Partial views are small but there can be many of them, and since the sg
> list space for them is allocated pessimistically, we can save some slab by
> trimming the unused tail entries.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> ---
> On the other hand with the diminishing importance of mmap ggtt do we
> expect to see partial views in use these days?

We have over a decade of hw that depends on them, and mesa is still not
entirely rid of them (and there's always i915_dri.so). So partials are
very much sticking around for a long time to come.

> ---
>  drivers/gpu/drm/i915/i915_drv.h     | 2 ++
>  drivers/gpu/drm/i915/i915_gem.c     | 2 +-
>  drivers/gpu/drm/i915/i915_gem_gtt.c | 2 ++
>  3 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 8624b4bdc242..0fe22263aa9b 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2323,6 +2323,8 @@ static inline struct scatterlist *__sg_next(struct scatterlist *sg)
>              (((__iter).curr += PAGE_SIZE) >= (__iter).max) ?           \
>              (__iter) = __sgt_iter(__sg_next((__iter).sgp), false), 0 : 0)
>  
> +bool i915_sg_trim(struct sg_table *orig_st);
> +
>  static inline unsigned int i915_sg_page_sizes(struct scatterlist *sg)
>  {
>         unsigned int page_sizes;
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index db9688d14912..7156ddef178b 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2491,7 +2491,7 @@ void __i915_gem_object_put_pages(struct drm_i915_gem_object *obj,
>         mutex_unlock(&obj->mm.lock);
>  }
>  
> -static bool i915_sg_trim(struct sg_table *orig_st)
> +bool i915_sg_trim(struct sg_table *orig_st)
>  {
>         struct sg_table new_st;
>         struct scatterlist *sg, *new_sg;

You can spare me having to move a few lines later if you move this to
i915_scatterlist_utils.c?

> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index f6c7ab413081..1ec721c20581 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -3835,6 +3835,8 @@ intel_partial_pages(const struct i915_ggtt_view *view,
>                 count -= len >> PAGE_SHIFT;
>                 if (count == 0) {
>                         sg_mark_end(sg);
> +                       i915_sg_trim(st); /* Drop any unused tail entries. */
> +

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris